In a wireless communication system, the inter-symbol interference (ISI) exists in received signals due to the common multi-path fading effect. To remove the ISI, a receiver is generally provided with an equalizer that needs information of channel impulse response (CIR) to operate, and therefore estimation of the CIR plays an important character in a mobile radio system.
In addition, OFDM, an important communication technology in the wireless communication field, is mainly for increasing the data transmission rate. For example, the data transmission rate in IEEE 802.11a using the OFDM technology reaches up to 54 Mbps, compared to the transmission rate of only 11 Mbps in IEEE 802.11b without the OFDM technology. Therefore, it is an important subject as how to effectively estimate CIR of an OFDM system to remove ISI, so as to fully exert a characteristic of high transmission rate of the OFDM technology. In the OFDM system, channel estimation, i.e., CIR estimation, is normally realized via pilot symbols that are known to a transmitter and a receiver.
Refer to FIG. 1 showing a block diagram of a conventional OFDM channel estimation apparatus comprising a 4096-sampling-point inverse fast Fourier transformation (IFFT) unit 101, a mirror image rejection unit 102, a 4096-sampling-point fast Fourier transformation (FFT) unit 103, and a frequency-domain response smoothing unit 104.
The 4096-sampling-point IFFT unit 101 performs 4096-sampling-point IFFT operation on a preliminary frequency channel response {tilde over (H)}(k) to generate a time-domain CIR {tilde over (h)}(n). Refer to FIG. 2 showing a frequency and time distribution comprising 4096 symbols {tilde over (h)}(0), {tilde over (h)}(1), {tilde over (h)}(2), . . . , {tilde over (h)}(4095). The conventional OFDM channel comprises 4096 sub-carriers, of which one in every 8 sub-carriers comprises a pilot sub-carrier for carrying a pilot symbol, and the remaining sub-carriers (other than the pilot sub-carriers) referred to as data sub-carriers are for carrying data symbols. That is, in the 4096 sub-carriers, there are 512 pilot sub-carriers for carrying pilot symbols, and there are 3584 data sub-carriers for carrying data symbols. FIG. 3 shows a time-frequency plane of 17×4096 symbols transmitted in a conventional OFDM channel. Estimation of a preliminary frequency-domain channel response {tilde over (H)}(k) is obtained by performing the least square difference algorithm on a frequency-domain transmitting value and a frequency-domain receiving value of a pilot symbol. That is, the preliminary frequency-domain channel response {tilde over (H)}(k) only has algorithm values at a frequency k corresponding to pilot sub-carries, and frequency channel response values corresponding to the remaining data sub-carriers are equal to zero. The 512 pilot symbols in the 4096 sub-carriers are distributed in two types, an even type and an odd type, alternatively. Please refer to FIG. 3, the even type is the horizontal row with a leading black circle as the following:    ●◯◯◯◯◯◯◯●◯◯◯◯◯◯◯●x●x●x●x,    and the odd type is the horizontal row with a leading while circle as the following:    ◯◯◯◯●◯◯◯◯◯◯◯●◯◯◯x●x●x●x●.The black circle ● represents pilot symbols. Pilot sub-carriers corresponding to the pilot symbols are at positions 0, 8, 16, 24 . . . 4088 in the even type and 4, 12, 20, 28 . . . 4092 in the odd type. The while circle ◯ represents data symbols, and the mark x represents repeating symbols as the left side. Therefore, the preliminary frequency-domain channel response {tilde over (H)}(k) may be an even frequency-domain channel response or an odd frequency-domain channel response according to the distribution of pilot symbols in 4096 sub-carriers. The even frequency-domain channel response and the odd frequency-domain channel response are alternatively transmitted to the 4096-sampling-point IFFT unit 101.
The mirror image rejection unit 102 maintains first and last 256 time-domain CIR values of the 4096 time-domain CIR {tilde over (h)}(n), and filters the mirror image signals, i.e., the remaining 3584 time-domain CIR values, to generate a time-domain CIR {tilde over (h)}w(n) as:
                    h        ~            w        ⁡          (      n      )        =      {                                                                      h                ~                            ⁡                              (                n                )                                      ,                                                n            =                                          [                                  0                  ,                  255                                ]                            ⋃                              [                                  3840                  ,                  4095                                ]                                                                                      0            ,                                                n            =                          [                              256                ,                3839                            ]                                          The FFT unit 103 performs 4096-sampling-point FFT operation on the time-domain CIR {tilde over (h)}w(n) to generate a frequency-domain channel response Ĥ(k), where k is equal to 0 to 4095.
The frequency-domain response smoothing unit 104 performs arithmetic averaging operation according to 17 groups of the frequency-domain channel responses, i.e., Ĥ1(k), Ĥ2(k), . . . , Ĥ17(k), to generate a re-estimated frequency-domain channel response ĤS(k) represented by:
                              H          ^                S            ⁡              (        k        )              =                  1        17            ⁢              (                                                            H                ^                            1                        ⁡                          (              k              )                                ,                                                    H                ^                            2                        ⁡                          (              k              )                                ,          …          ⁢                                          ,                                                    H                ^                            17                        ⁡                          (              k              )                                      )              ,where k is equal to 0 to 4095.
In the conventional OFDM channel estimation apparatus, only 512 sampling values of the 4096-sampling-point IFFT operation are non-zero, and others are equal to zero. Therefore, the 4096 IFFT operation is in fact large in scale that lacks of efficiency and needs to be improved. According to the present invention, a low-cost solution for reducing IFFT operation scale according to characteristics of China multimedia mobile broadcasting (CMMB) OFDM systems is provided to maintain a same channel estimation efficiency as when the operation scale is small.
In view of the problem, a novel algorithm for channel estimation is provided by the invention. The algorithm implements smaller scale IFFT operation to adaptively adjust operation formulas for processing the preliminary frequency-domain channel response to reach the same performance, so as to significantly reduce the overall operation scale.